Glutathione S-transferases, the enzymes which catalyze the conjugation of glutathione with an enormous variety of compounds that bear an electrophilic center, are also active binding proteins. The enzymes from rat and human liver have been purified and are available for testing with a variety of ligands. Among such ligands must be included substrates, non-reactive compounds such as bilirubin, and a third class which is sufficiently reactive that covalent bands with protein are formed.